The Scent of Blood: If an opposing character has taken damage this turn, modify Zuvembie's attack, speed, and damage values by +1.

Zombie Plague: When Zuvembie KO's an opposing character with an attack, you may place a AS #006 Zuvembie on its starting line in its square and add it to your force.

What is the duration of the modifier granted by The Scent of Blood?
This current turn. Any time you would go to check the Zuvembie's combat values and look for modifiers, just ask yourself, "Was an opposing character damaged THIS TURN?"

If not, there is no modifier. If there was, modify away.

If I Mind Control an opponent's Zuvembie, and manage to KO a character with that Controlled Zuvembie, do I get to add a new Zuvembie to my force?
Yes. It was your Zuvembie when the character was KO'd, so the newly created Zuvembie is also yours.

Note, though, that this probably won't work unless you have some of your own Zuvembies to use. Absolutely nothing forces your opponent to let you use his or her extra characters. You can try asking politely, but they are free to refuse.